Double feed detection method and device

ABSTRACT

A double feed detection method for detecting a double feed of a sheet-like detection object transported through a processing unit has an ultrasonic transmitter and an ultrasonic receiver positioned so as to sandwich a transport path along which the sheet-like detection object is transported so as to detect a double feed of the detection object. The double feed detection method has a step of forming a gap between the sheet-like detection objects where the sheet-like detection objects overlap when a double feed of the sheet-like detection objects occurs. Forming the gap between the sheets enhances an attenuation of an intensity of a response signal whenever a double feed occurs. As a result, a distinction between the intensity of the response signal when a double feed has occurred and when no double feed has occurred is sharpened and thus more easily detected, improving the double feed detection accuracy.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to a double feeddetection method and device, and more particularly, to a double feeddetection method and device in which an ultrasonic sensor is used todetect double feeding of a sheet-like detection object in the course oftransport.

[0003] 2. Description of Related Art

[0004] In the case of an image scanner or an optical character reader(OCR), the image scanner or OCR reads lettering and the like printed onboth sides of the paper after the paper has been separated into singlesheets. However, if by some mistake two sheets remain unseparated andare transported onward in that double-fed state, then the image scanneror OCR is unable to read the two overlapped surfaces. Moreover, if acertain number of sheets are to be read in order, then such doublefeeding disrupts the accuracy of the count, which is undesirable. As aresult, it is important not only to prevent the occurrence of double-fedsheets but also to detect accurately such double feeds when they occurand to deal with them promptly.

[0005] It should be noted that the detection of double feeds is notlimited to sheets of paper, as described above, but may encompass anysheet-like object which should be fed through a transport unit one sheetat a time, such as plastic, metal, and so forth.

[0006] As conventional methods for detecting a double feed, there existthose that use optical sensors and those that use ultrasonic sensors.However, in the case of optical sensors, the condition of the print onthe printed surfaces of the paper can change the amount of lightreflected from or passing through the paper, and so for this reason theultrasonic sensor method is more commonly used because the detectionvalues do not change depending on the condition of the print.

[0007]FIG. 1 is a lateral cross-sectional view of a conventional doublefeed detection device utilizing an ultrasonic sensor, in this caseJapanese Laid-Open Patent Application No. 6-49537.

[0008] The double feed detection device 1 a is provided with a mesh-likesupport stand 2 through which ultrasonic waves pass. Sheets of paper 3 aare carried along a transport route (not shown in the diagram) on a topsurface of the support stand 2. Placed a predetermined distance apartand sandwiching the upper surface of the support stand 2 are ultrasonictransmitter element 4 a and ultrasonic receiver element 4 b, disposedopposite each other. The ultrasonic receiver element 4 b is mounted onan internal holder 5 b of a cooling jacket case 5 a, the internal holder5 b having a skirt 5 c on a bottom part thereof.

[0009] An annular nozzle 6 is provided in a space between the skirt 5 cand a jacket outer wall 5 d, such as to allow a flow of air, that is, anair curtain 7, to be generated so as to press the paper 3 a onto thesupport stand 2 and at the same time to shield the interior of thestructure from externally generated heat, thus serving to eliminatefluctuation in detection values due to changes either in temperature orin the flatness of the surface of the paper 3 a due to shaking.

[0010] Additionally, though not shown in the diagrams, a sheetmulti-feed detection device according to Japanese Laid-Open PatentApplication No. 6-49567, comprising a pair of sheet retainers locatednear and pinching the lateral sides of a portion of a sheet feed paththrough which ultrasonic waves pass as well as detecting means fordetecting when a sheet is retained by one or the other of the retainingmeans, is configured to detect a multi-feed state based on anattenuation of the ultrasonic sound waves when it is detected that asheet is retained by one or the other of the retaining means.

[0011] As will be appreciated by those of skill in the art, if the sheetof paper flutters, then the detection signal also becomes unreliable. Inthe structure described above, the sheet is caught by one or the otherof the sheet retainers and detection takes place without the sheetfluttering, so an erroneous reading can be prevented.

[0012] In both of the above-described cases, the devices involved aredesigned to ensure accurate detection of a double feed by for exampleeliminating the erroneous readings that are caused by fluctuation of thepaper.

[0013] As another example of the conventional art there is the doublefeed detection device according to Japanese Laid-Open Patent ApplicationNo. 5-40030, shown in FIG. 2.

[0014] The double feed detection device 1 b has an ultrasonic soundtransmitter 8 a that transmits ultrasonic sounds of a predeterminedintensity and an ultrasonic sound receiver 8 b disposed opposite eachother and sandwiching the transport path through which the sheet-likedetection object 3 b passes. The intensity of the ultrasonic soundsdetected by the ultrasonic sound receiver 8 b are compared to apredetermined reference value, and if the detected intensity is at orbelow the reference value then it is determined that the detectionobject 3 b is double fed and a signal indicating such double feed isoutput by a double feed discrimination circuit 9.

[0015] The above-described detection principle is based on the fact thatthe intensity of the ultrasonic sound that passes through the detectionobjects 3 b when two such sheet-like detection objects 3 b pass betweenthe ultrasonic sound transmitter 8 a and the ultrasonic sound receiver 8b is virtually constant regardless of the material or the thickness ofthe detection object 3 b, which means that the need to make an initialadjustment each time the material changes can be eliminated.

[0016] However, the double feed detection device according to JapaneseLaid-Open Patent Application No. 5-40030 differs from the first twoconventional examples described above and has a disadvantage in that,when detecting a double feed of sheet-like detection objects ofdifferent materials and thicknesses, the detection sensitivity, that is,the limit at which detection can be conducted without adjusting theultrasonic sensor, is reduced.

SUMMARY OF THE INVENTION

[0017] Accordingly, it is a general object of the present invention toprovide an improved and useful double feed detection method and device,in which the above-described disadvantage is eliminated and double feeddetection sensitivity is improved.

[0018] Another more specific object of the present invention is to forman air gap between double-fed sheets so as to greatly reduce the amountof ultrasound that passes through the two as compared to when there isonly one sheet present or two sheets stuck together without any air gapin between, thereby heightening the contrast between one sheet fedproperly and two sheets that are double fed.

[0019] The above-described objects of the present invention are achievedby a double feed detection method for detecting a double feed of asheet-like detection object being transported through a processing unit,an ultrasonic transmitter and an ultrasonic receiver being positioned soas to sandwich a transport path along which the sheet-like detectionobject is transported so as to detect a double feed of the detectionobject, the double feed detection method comprising a step of forming agap between the sheet-like detection objects where the sheet-likedetection objects overlap when a double feed of the sheet-like detectionobjects occurs.

[0020] Additionally, the above-described objects of the presentinvention are also achieved by a double feed detection device fordetecting a double feed of a sheet-like detection object beingtransported through a processing unit using an ultrasonic sensor todetect a double feed of the detection object, the double feed detectiondevice comprising:

[0021] an ultrasonic transmitter and an ultrasonic receiver positionedso as to sandwich a transport path along which the sheet-like detectionobject is transported; and

[0022] a gap-forming mechanism that forms a gap between the sheet-likedetection objects where the sheet-like detection objects overlap when adouble feed of the sheet-like detection objects occurs.

[0023] Additionally, the above-described objects of the presentinvention are also achieved by an optical form reader comprising:

[0024] a scanner including a part that optically reads informationstored on a form; and

[0025] a double feed detection device for detecting a double feed of aplurality of forms being transported through a processing unit using anultrasonic sensor to detect a double feed of the detection object,

[0026] the double feed detection device comprising:

[0027] an ultrasonic transmitter and an ultrasonic receiver positionedso as to sandwich a transport path along which the forms aretransported; and

[0028] a gap-forming mechanism that forms a gap between the forms wherethe forms overlap when a double feed of the forms occurs.

[0029] Other objects, features and advantages of the present inventionwill become more apparent from the following detailed description whenread in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a lateral cross-sectional view of a conventional doublefeed detection device;

[0031]FIG. 2 is a structural diagram of another conventional double feeddetection device;

[0032]FIG. 3 is a lateral view of an optical scanner equipped with adouble feed detection device according to one embodiment of the presentinvention;

[0033]FIG. 4 is a flow chart illustrating an operation of the opticalscanner shown in FIG. 3;

[0034]FIG. 5 is a side view of a double feed detection device accordingto a first embodiment of the present invention;

[0035]FIG. 6 is a circuit structure diagram of the double feed detectiondevice shown in FIG. 5;

[0036]FIGS. 7A and 7B are diagrams illustrating the double feeddetection device shown in FIG. 5, in which FIG. 7A is a plan view of thedouble feed detection device shown in FIG. 5 and FIG. 7B shows a geararrangement of the double feed detection device shown in FIG. 5;

[0037]FIGS. 8A, 8B, 8C and 8D are diagrams showing sample changes withtime in transmitted sound volume, output voltage inversion value,inverted output voltage peak hold value and comparator output,respectively;

[0038]FIG. 9 is a lateral view of a double feed detection deviceaccording to a second embodiment of the present invention; and

[0039]FIG. 10 is a plan view of a double feed detection device accordingto a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] A description will now be given of embodiments of the presentinvention, with reference to the accompanying drawings. It should benoted that identical or corresponding elements in the embodiments aregiven identical or corresponding reference numbers in all drawings, withdetailed descriptions of such elements given once and thereafteromitted.

[0041] Additionally, though the following description refers to a doublefeed and its detection, it should be understood that the term “doublefeed” is meant to refer to any overlapping of two or more sheet-likedetection objects, and as such is not limited to situations involving anoverlap (either partial or complete) between two sheets but includesalso overlaps (partial or complete) of three or more sheets.

[0042] The term ultrasonic applies to sound waves above the range ofhuman hearing, that is, frequencies in the range of 20-220 kHz. It isknown that a portion of such sound waves are reflected at a boundarylayer between media of different acoustic impedances. Acoustic impedanceis a product of the speed of sound waves through the medium and thedensity of the medium, and therefore the difference in acousticimpedance between a solid such as paper on the one hand and air on theother, whose sound speeds and densities differ so sharply, is verygreat. By taking advantage of this difference and forming an air gapbetween doubled sheets of paper, approximately 99% of the ultrasonicwaves that pass through the first sheet and the subsequent air gap arethen reflected back from the surface of the second sheet, meaning thatthe amount of ultrasound that passes through the two sheets is greatlyreduced as compared to when there is only one sheet present or twosheets stuck together without any air gap in between, therebyheightening the contrast between one sheet fed normally and two sheetsthat are double fed.

[0043] By contrast, as described above, the volume of ultrasound thatpasses through one sheet and the volume of ultrasound that passesthrough two sheet that are stuck together is virtually identical.

[0044] Moreover, in the case of frequencies in the audible range, themaximum above-described effect cannot be obtained.

[0045]FIG. 3 is a lateral view of an optical scanner equipped with adouble feed detection device according to one embodiment of the presentinvention.

[0046] As shown in FIG. 3, a double feed detection device 10 isinstalled in a optical form reader 12, the optical form reader 12comprising a hopper 14, a pick-up roller 16, a form separator 18, thedouble feed detection device 10, a transport roller 20, a transport path22, a stacker 24, a form front-surface optical reading unit 26 and aform back-surface optical reading unit 28.

[0047] A description will now be given of an operation of the opticalform reader 12, with reference to FIG. 3 and FIG. 4.

[0048]FIG. 4 is a flow chart illustrating an operation of the opticalscanner shown in FIG. 3.

[0049] A multiplicity of forms constituting the sheet-like detectionobjects 30 is set in the hopper 14. The pick-up roller 16 takes theforms 30 one sheet at a time starting from the top of the stack andfeeds the forms 30 onward in an X1 direction to the form separator 18 ina step S1.

[0050] If for some reason the forms 30 are double fed, then they areseparated by the form separator 18 in a step S2. The forms 30 are thensent onward from the form separator 18 one at a time.

[0051] However, it may happen that two forms are stuck together, forexample by static electricity, and in that unseparated state emittedfrom the form separator 18. In this case, the double feed detectiondevice 10 detects the presence of double-fed forms 30 in a double feedcheck step S3, and flashes an alarm indicator and causes the opticalform reader to stop in an stop or alarm step S4.

[0052] On the other hand, a properly separated single form 30 passesthrough the double feed detection device 10, is transported by thetransport roller 20, and the lettering or other information written onthe front and back of the form 30 is read by the form front-surfaceoptical reading unit 26 and the form back-surface optical reading unit28 in a step S5, and the form 30 thereafter transported along thetransport path 22 to the stacker 24.

[0053] The next form 30 is then taken from the hopper 14 and undergoesthe same processing as described above.

[0054] Descriptions will now be given in order of three differentembodiments of the double-feed detection device 10 according to thepresent invention, with reference to the accompanying drawings.

[0055]FIG. 5 is a side view of a double feed detection device accordingto a first embodiment of the present invention.

[0056] As shown in FIG. 5, a double-feed detection device 10 a accordingto the first embodiment of the present invention comprises an ultrasonicsensor 32 transmitter 32 a and an ultrasonic sensor 32 receiver 32 bdisposed opposite each other so as to sandwich the transport path 22 onthe X1 side of the form separator 18. Additionally, paired rollers 34 a,34 b and 36 a, 36 b are provided at upstream and downstream sides,respectively, of the ultrasonic sensor 32.

[0057] The ultrasonic sensor 32 transmitter 32 a is a transmitter, andthe receiver 32 b is a microphone. Ultrasonic sound waves of apredetermined intensity are generated from the transmitter 32 a,attenuate after passing through the form 30 interposed in a path ofpropagation of the ultrasonic sound waves, and in that attenuated stateare collected by the receiver 32 b.

[0058]FIG. 6 is a circuit structure diagram of the double feed detectiondevice shown in FIG. 5.

[0059] The reception signal output from the receiver 32 b is amplifiedby an amplifier 38, after which it is compared to a cutoff signal (areference signal) by a comparator 40, and that output then sent to amicroprocessor unit (MPU) 42. If attenuation meets or exceeds apredetermined value, then an alarm is indicated at an operating panel 43by a signal sent from the MPU 42. At the same time, a stop signal isgenerated by a control unit 44 by a signal sent from the MPU 42. Thegeneration of the stop signal causes the optical form reader 12 to stop.

[0060]FIGS. 7A and 7B are diagrams illustrating the double feeddetection device shown in FIG. 5, in which FIG. 7A is a plan view of thedouble feed detection device shown in FIG. 5 and FIG. 7B shows a geararrangement of the double feed detection device shown in FIG. 5.

[0061] Rollers 34 a, 34 b and rollers 36 a, 36 b are driven by a singlemotor in order to make the device more compact, with the motor and gearslaid out in the configuration shown for example in FIGS. 7A and 7B. Themotor 46 is directly linked to a gear 48 a, and the gear 48 a is linkedin order to gears 48 b, 48 c and 48 d in one direction and, in the otherdirection, is linked in order to gears 48 e, 48 f, 48 g 48 h, 48 i, and48 j. Gear 48 h and roller 34 a, and gear 48 b and roller 34 b, and gear48 j and roller 36 a, and gear 48 d and roller 36 b are each provided onthe same shaft. In this case, the gears 48 h, 48 b and 48 d have thesame diameter, with the gear 48 j having a diameter that is slightlylarger than that of gears 48 h, 48 b and 48 d.

[0062] The rollers 34 a, 34 b and the rollers 36 a, 36 b are in thiscase all of the same diameter. Therefore, when the gears are rotated bythe motor 46, only the roller 36 a, which is directly linked to the gear48 j (the gear 48 j having the slowest rotation speed), rotates moreslowly than the other rollers 34 a, 34 b and 36 b.

[0063] A description will now be given of the double feed detectiondevice 10 a having the above-described structure according to the firstembodiment of the present invention.

[0064] As described previously, the multiplicity of forms 30 in thehopper 14 are transported one at a time in the X1 direction to theseparator 18 by the pick-up roller 16.

[0065] It occasionally happens that two forms 30 are stuck together andsent onward in that unseparated state. In such a case, the double-fedforms are separated by the form separator 18. Then, the separated forms30 are sent onward one at a time from the form separator 18.

[0066] However, if, for example, a plurality of forms 30 are stucktogether by static electricity and fail to be separated by the formseparator 18, then these unseparated forms may be mistakenly sent onwardfrom the form separator 18 to the double feed detection device 10 a.

[0067] In this case, the coefficient of friction between the form 30 andthe rollers 36 a and 36 b is set to be larger than the coefficient offriction between the double-fed forms 30. As a result, a form 30 b thatcontacts the roller 36 b (the bottom form shown in FIG. 4) starts toslip with respect to the form 30 a on top of the bottom form 30 b, suchthat the rotation of the roller 36 b transports the bottom form 30 b atan ordinary transport speed in the X1 direction onward to a nextprocessing stage. At the same time, the top from 30 a, which contactsthe roller 36 a, is sent onward by the roller 36 a in the X1 directionto the next processing stage at a transport speed slightly less thanthat of the roller 36 b.

[0068] Accordingly, the top form 30 a, which is sent onward at atransport speed that is slower than that of the bottom form 30 b,gradually forms a bulge A upstream of the roller 36 a, that is, on an X2side. If, for example, the ratio of the rotation speed of the roller 36a to the roller 36 b is 30:29, then when the lower form 30 b hasadvanced 30 mm the upper form 30 a has formed a bulge A of 1 mm.Accordingly, at the portion of the bulge A, a slight gap C is formedbetween the upper and lower forms 30 a and 30 b. Thus the rollers 34 a,34 b, 36 a and 36 b function as a gap-forming mechanism, forming a gapbetween double-fed forms 30 a and 30 b. In other words, the roller 34 aacts as a brake and the roller 34 b acts as a transport part. It shouldbe noted that, depending on the dimensions of the form, when thedistance between the form separator 18 and the rollers 32 a and 34 b isshort, the rollers 32 a and 34 b can be eliminated so that just therollers 36 a and 36 b form the gap-forming mechanism.

[0069] Thus, a double feed of the forms 30 is detected by the ultrasonicsensor 32 using the acoustic pattern shown in FIGS. 8A, 8B and 8C.

[0070]FIGS. 8A, 8B, 8C and 8D are diagrams showing sample changes withtime in transmitted sound volume, output voltage inversion value,inverted output voltage peak hold value and comparator output,respectively.

[0071]FIG. 8A shows the amount of sound passing through the detectionobject (transmitted sound volume, measured in decibels) on the verticalaxis and time (measured in seconds) on the horizontal axis.

[0072] If for example a single N-1 form 30 passes the ultrasonic sensor32 detection point, then during a time period t1-t2 (the time it takesfor the form 30 to pass through the detection point), the transmittedsound volume, that is, the amount of sound passing through the form 30,shows an attenuation of W1. (For convenience of description, theun-attenuated volume of sound transmitted when no form 30 passes throughthe detection point at a time prior to t1 is taken as the referenceacoustic level).

[0073] During the time t2 to t3, that is, during the time required for asucceeding N form 30 to reach the detection point at time t3, thetransmitted sound volume hardly attenuates at all.

[0074] When the N form 30 passes through the ultrasonic sensor 32detection point during time t3-t4, the amount of sound passing throughthe N form 30 shows an attenuation of W2 as shown in FIG. 8A. In thiscase, slight variations in the thicknesses of the N-1 and N forms 30 aswell as in their surface conditions lead to a slight difference in theamount of sound passing through the forms during detection, a differenceindicated as Δ in FIG. 8A.

[0075] When, for some reason, at a time t5, forms N+1 and N+2 reach theultrasonic sensor 32 detection point in a double-fed state, the passageof these two overlapping forms 30 through the detection point at thesame time results in an attenuation of the volume of sound passingthrough the forms.

[0076] At this time, the upper and lower forms 30 are stuck togethercompletely, with no gap therebetween, and in such state pass through theultrasonic sensor 32 detection point at time intervals shown as t5-t6 inFIG. 8A, causing an attenuation in transmitted sound volume shown as W3in FIG. 8A.

[0077] As can be appreciated from FIG. 8A, the attenuation W3 obtainedat time interval t5-t6, obtained when the double-fed forms 30 pass theultrasonic sensor 32 detection point with no gap formed between theforms, does not differ greatly from the attenuations W1, W2, obtainedwhen N-1 and N forms 30 pass the detection point as single sheets.

[0078] By contrast, when the rollers 36 a, 36 b that function asgap-forming means form a gap C between the upper and lower double-fedsheets 30 a, 30 b, and when that portion of the double-fed forms 30 a,30 b in which the gap C is formed passes the detection point at timeintervals t6-t7 and again t8-t9, the sound transmitted from thetransmitter 32 a and shown as level S1 in FIG. 5 passes through thelower sheet 30 b to become a level S2 as shown in FIG. 5, furtherpassing through the air gap formed between the upper and lower forms 30a, 30 b to reach a lower surface of the upper form 30 a. At this point aportion of the sound transmitted from the transmitter 32 a is reflectedfrom the lower surface of the upper form 30 and attenuates sharply as aresult, and in that attenuated state the sound passes through the upperform 30 a and is further attenuated thereby, attaining a level indicatedas S3 in FIG. 5. As a result, the sound reaches the receiver 32 b in agreatly attenuated state indicated in FIG. 8A as W4.

[0079] Accordingly, the attenuation W4 at the portion of the double-fedforms where the air gap C is formed is much greater than either theattenuations W1, W2 obtained with mere single forms 30 having slightvariations in thickness and surface characteristics or the attenuationW3 obtained when two forms 30 are double-fed without any air gap formedin between. Consequently, a state in which sheets are double fed can bedistinguished with a high degree of accuracy from a state in whichsingle sheets are fed properly. As a result, by setting an appropriatethreshold level L1 for distinguishing the former state from the latter(appropriate insofar as the effects of external disturbances on the unitare taken into account), whenever the attenuation W exceeds thethreshold level L1 (in terms of FIG. 8, whenever the transmitted soundvolume as an absolute value drops below the threshold value L1, a doublefeed can be detected. Upon detection a detection signal can be used totrigger an alarm indicator and to stop the optical form reader 12 asappropriate.

[0080] In the case described above, in place of the output voltagesignals of the ultrasonic sensor 32 (the output voltage signalscorresponding to the transmitted sound volume), the output voltagesignals can be inverted (as shown in FIG. 8B) by an inversion amplifyingcircuit, the inverted voltage signal can be held at a peak value for apredetermined time period by a peak hold circuit, and when that peakhold value exceeds a slice level L2 as shown in FIG. 8C, a comparatoroutput can be turned ON in a FIG. 8D. In so doing, instantaneous changesin the output signals of the ultrasonic sensor 32 generated by a doublefeed can be detected quickly and accurately.

[0081] Next, a description will be given of a double feed detectiondevice 10 b according to a second embodiment of the present invention,with respect to FIG. 9.

[0082]FIG. 9 is a lateral view of the double feed detection deviceaccording to the second embodiment of the present invention.

[0083] The double feed detection device 10 b comprises a transmitter 32a and a receiver 32 b (together comprising an ultrasonic sensor 32)provided above and below a transport path surface of the transport path22 on the downstream side (X1 side) of the form separator 18.Additionally, a pad-like, curved panel member 50 and a roller 52 aredisposed opposite each other above and below the transport path 22downstream of the ultrasonic sensor 32.

[0084] According to the double feed detection device 10 b describedabove, the pad-like member 50 performs the same role as the roller 36 aof the first embodiment, and so the same effects as with the firstembodiment of the present invention can be obtained. In this case, theuse of the pad-like member 50 in place of the roller 36 a simplifies thedesign of the device. It should be noted that when this pad-like memberis made of an electrically conductive material it is possible to removeany static electric charge residing on the forms 30 during theirtransport, which removal is desirable.

[0085] Next, a description will be given of a double feed detectiondevice 10 c according to a third embodiment of the present invention,with respect to FIG. 10.

[0086]FIG. 10 is a plan view of the double feed detection deviceaccording to the third embodiment of the present invention.

[0087] The double feed detection device 10 c has pairs of rollers 54 a,54 b and 56 a, 56 b disposed opposite each other along both sides aswell as above and below the transport path 22 at a position downstream(in the X1 direction) of the form separator 18. The transmitter 32 a andthe receiver 32 b of the ultrasonic sensor 32 are positioned downstreamin the X1 direction of the pair of paired rollers.

[0088] The rollers 54 a, 56 a are arranged so that their rotation shaftsare at right angles to the transport direction (the Y direction). Bycontrast, however, rollers 54 b and 56 b are positioned so that theirrotation shafts are offset with respect to the centerlines of therollers 54 a and 56 a by a slight angle indicated in FIG. 10 as θ. Itshould be noted that in FIG. 10 angle θ has been exaggerated forpurposes of illustration only, and that in actuality the angle is smalland set experimentally, that is, is varied according to the quality ofthe forms 30. The rollers 54 a and 56 a are provided on the same shaftand form the drive side, that is, the transport part. The rollers 54 b,56 b press the lateral edges of the form 30 inward toward a center ofthe form 30.

[0089] When the forms 30 are double fed, the rollers 54 a, 54 b and 56a, 56 b work to press the lateral edges of the forms 30 toward thecenter of the forms 30, such that the bottom form 30 b acquires a bulge,thereby opening a gap between it and the top form 30 a in which no bulgeis formed.

[0090] As described above, the double feed detection device 10 caccording to the third embodiment of the present invention also achievesthe same effect as the double feed detection device 10 a according tothe first embodiment of the present invention as described above.

[0091] It will be appreciated by those of skill in the art that thepresent invention is not limited to the detection of double feeds ofpaper as described above but can be applied to virtually anysheet-shaped detection object and thus can include paper, plastic andmetal detection objects.

[0092] Moreover, it should be noted that, although the embodimentsdescribed above make reference to an optical form reader, the presentinvention is not limited to such embodiments but can be adapted to anyprocessing unit accommodating sheet-like detection objects.

[0093] The above description is provided in order to enable any personskilled in the art to make and use the invention and sets forth the bestmode contemplated by the inventors of carrying out the invention.

[0094] The present invention is not limited to the specificallydisclosed embodiments, and variations and modifications may be madewithout departing from the scope and spirit of the present invention.

[0095] The present application is based on Japanese Priority ApplicationNo. 2000-150335, filed on May 22, 2000, the entire contents of which arehereby incorporated by reference.

What is claimed is:
 1. A double feed detection method for detecting adouble feed of a sheet-like detection object being transported through aprocessing unit, an ultrasonic transmitter and an ultrasonic receiverbeing positioned so as to sandwich a transport path along which thesheet-like detection object is transported so as to detect a double feedof the detection object, the double feed detection method comprising astep of forming a gap between the sheet-like detection objects where thesheet-like detection objects overlap when a double feed of thesheet-like detection objects occurs.
 2. A double feed detection devicefor detecting a double feed of a sheet-like detection object beingtransported through a processing unit using an ultrasonic sensor todetect a double feed of the detection object, the double feed detectiondevice comprising: an ultrasonic transmitter and an ultrasonic receiverpositioned so as to sandwich a transport path along which the sheet-likedetection object is transported; and a gap-forming mechanism that formsa gap between the sheet-like detection objects where the sheet-likedetection objects overlap when a double feed of the sheet-like detectionobjects occurs.
 3. The double feed detection device as claimed in claim2 , wherein the gap-forming mechanism comprises: a transport part thatsandwiches the sheet-like detection object at a trailing end of thetransport path in a direction of transport of the sheet-like detectionobject by a transporting mechanism along the transport path; and a brakepart, the brake part being a resistive element that reduces a speed withwhich the sheet-like detection object is transported by the transportingmechanism, the brake part generating a bulge, in the direction oftransport of the sheet-like detection object along the transport path,in the sheet-like detection object that contacts the brake part.
 4. Thedouble feed detection device as claimed in claim 2 , wherein thegap-forming mechanism is provided at a trailing end of the transportpath in the direction of transport of the sheet-like detection object bythe transporting mechanism along the transport path and the gap-formingmechanism includes a transport part that sandwiches the sheet-likedetection object at both lateral edges, the double feed detection devicefurther comprising a part that presses the lateral edges of thesheet-like detection object inward toward a center of the sheet-likedetection object, such that a width of a gap formed between double fedsheet-like detection objects is at a right angle to the direction oftransport of the sheet-like detection object.
 5. The double feeddetection device as claimed in claim 3 , wherein the brake part and thetransport part comprise at least one pair of rollers that sandwich thesheet-like detection object between them, the roller that constitutesthe brake part having a rotation speed that is slower than a rotationspeed of the roller that constitutes the transport part.
 6. The doublefeed detection device as claimed in claim 3 , wherein the brake part isa pad-like member that scrapes one surface of the sheet-like detectionobject.
 7. An optical form reader comprising: a scanner including a partthat optically reads information stored on a form; and a double feeddetection device for detecting a double feed of a plurality of formsbeing transported through a processing unit using an ultrasonic sensorto detect a double feed of the detection object, the double feeddetection device comprising: an ultrasonic transmitter and an ultrasonicreceiver positioned so as to sandwich a transport path along which theforms are transported; and a gap-forming mechanism that forms a gapbetween the forms where the forms overlap when a double feed of theforms occurs.